Color images can be captured and converted into a video signal, which can be transmitted to a display system, such as a television. The display system typically processes the input video signal and transmits the processed video signal to a display device, which reproduces the luminance and color of the images onto its display screen for a viewer. Typical display devices include liquid-crystal displays (LCD), cathode-ray tubes (CRT), and plasma display panels (PDP). Each display device employs complex mechanisms that take the video signal and reproduce the luminance and color properties of the image.
For example, the luminance and color reproduction of an LCD is characterized by the spectral power distribution (SPD) of a backlight, e.g., cold-cathode fluorescent light (CCFL) tubes or light-emitting diodes (LEDs), the transmission characteristics of the polarizers and color filters for each primary-color sub-pixel on the screen, and the transmission characteristics of the liquid crystal cells under different electrical field strengths controlled by each pixel data from the input video signal. The luminance and color reproduction of a CRT are characterized by the SPD of each kind of phosphor material of each primary-color dot in a triad on the screen and the electron emitting dynamics of the electron guns under different electrical field strengths on the grids controlled by the input video signal. The luminance and color reproduction of a PDP are characterized by the SPD of each kind of phosphor material of each primary-color sub-pixel on the screen and the electrical discharge dynamics of the dielectric layers among the electrodes with voltage differences controlled by each pixel data from the input video signal.
Important properties of a display device are characterized by the chromaticity values of the device's primary color components, the reference white point, and the device's power transfer function from input signal voltage to output luminance level. In a typical display device, luminance curves, gamma values, color chromaticity values, and color temperatures are set to predetermined nominal settings during the manufacturing process. With these settings, the ideal display device can accurately reproduce the luminance and color properties of the captured image to provide an enjoyable viewing experience for the viewer.
Nonetheless, most mass-produced display devices are less than ideal. The predetermined nominal settings are difficult to attain due to the display device's imperfections that arise during the manufacturing process, as well as the complex underlying physical mechanisms of the display devices themselves. Thus, for standard mass-produced display devices, where quality control during manufacturing is more lax in order to keep costs low, the predetermined nominal settings are not necessarily achievable. For instance, because the nominal settings are usually mismatched, it is common for standard display devices to exhibit undesirable gray level color deviations and color temperature shifts. Such gray-level color deviations cause the measured chromaticity values of a displayed white color to drift on a chromaticity diagram with different gray levels. Moreover, gray-level color deviations also cause the measured color temperature of a displayed white color to vary with different gray levels. These undesirable color deviations and color temperature shifts diminish the image quality of the display device.
Moreover, the actual, i.e., measured, luminance curve of the standard display device can also deviate from the predetermined power-law transfer function. Accordingly, the measured luminance curves, gamma values, color chromaticity values, and color temperatures of typical mass-produced display devices usually deviate from expected values, and in some instances, the values that should be constant, e.g., gamma values, are variable and curves that are supposed to be smooth, e.g., luminance curves, are uneven. For these reasons, standard mass-produced display devices often render non-ideal perceived image quality.
Accordingly, it is desirable to provide a method and system for improving the perceived image quality of color display devices. In particular, it is desirable to provide a method for automatically calibrating a color display device such that the output characteristics of the display device are substantially in line with the expected output characteristics of an ideal display device.